In a wind power plant it is known art to perform real-time control of the power output from each wind turbine or sub-station to comply with grid requirements. The control typically takes place from a central power plant controller unit in a sub-station and may involve control of active power, reactive power, frequency, current and/or voltage.
As wind power plants increase in size and power output, the consequences of failure in the control of the power plant becomes more severe.
New grid compliance codes pose requirements to reliable and deterministic power output from power plants in both normal operational scenarios and in case of grid failures inside or outside the wind power plant.
Wind power plants are more and more often relied on in relation to compensating the grid in case of grid failures outside the wind power plant.
It is therefore desirable to have a power plant control system that is highly reliable characterized in:                Very high reliability in the power plant controller system. By “very high reliability” is here understood that the power plant controller system has properties that with very high probability guarantee correct power plant control in normal operational scenarios, in single-failure scenarios and possibly also in double or multiple failure scenarios.        Very high reliability in the power plant communication network used for power plant control. By “very high reliability” is here understood that the communication network has properties that with very high probability guarantee delivery of correct data in both normal operational scenarios, single-failure scenarios and possibly also in double or multiple failure scenarios.        Hard real-time properties in the communication network between the power plant controller and/or power plant resource controllers and the connected wind turbines and sub-stations. By “hard real-time” is here understood that the communication network has properties that with very high probability guarantee delivery of data within a specified deadline in all load scenarios.        
The requirement for very high reliability in the power plant control is supported by fault-tolerance in the power plant control system and power plant communication system. The requirement is further supported by decentralized voting schemes, said decentralized voting schemes aiming at selecting the most reliable wind turbine set point value among a plurality of available wind turbine set point values.
Fault-tolerant control systems for wind power plants are typically implemented as redundant systems comprising duplicates of various critical plant modules/devices. In case a critical plant module/device fails its functionality is taken over by a similar plant module/device.
US 2009/0309360 and US 2009/0309361 discuss a method and a system for controlling a wind energy park. In US 2009/0309360 and US 2009/0309361 a main communication unit controls a number of prioritised control units. In case a given control unit with a given priority fails, the main communication unit selects a lower prioritised control unit to take over the functionality of the defective control unit.
It is a disadvantage of the method and system suggested in US 2009/0309360 and US 2009/0309361 that the main communication unit selects which control unit to take over in case another control unit breaks down or in any other way malfunctions. However, in case the main communication unit itself breaks down, no replacement unit is available.
Thus, the control method and the control system suggested in US 2009/0309360 and US 2009/0309361 can not be considered a fault-tolerant control method/system—at least not on the main communication unit level.
It may be seen as an object of embodiments of the present invention to provide highly reliable real-time power plant control in a wind power plant.